Mechanisms that minimize retinal impact of apolipoprotein E absence

Abstract

the blood, APOE is mainly associated with VLDLs, IDLs, and chylomicron remnants (2). In the brain and retina, however, the APOE-containing LPPs are different and have densities similar to those of HDLs (3, 4). APOE mediates lipid transfer by being a recognition ligand for specific cellsurface receptors, which bind and internalize the APOEcontaining LPPs (5) after these particles acquire cholesterol from cells with cholesterol excess. The APOE receptors belong to the LDL receptor (LDLR) family and include LDLR, VLDLR, LDLR-related protein 1 (LRP1), LRP1B, LRP2 (megalin), LRP4 (MEGF7), LRP5, LRP6, LRP8 (APOER2), and LRP11 (SORL1) (6) In humans, APOE exists in three isoforms (2, 3, and 4), which differ in their lipid-binding capacity, ability to integrate into LPP, and affinity for the receptors (7). APOE 4 is a risk factor for Alzheimer's disease, whereas APOE 2 is protective (8-10). Conversely, APOE 4 and 2 decrease and increase risks, respectively, for age-related macular Abstract Apolipoprotein E (APOE) is a component of lipid-transporting particles and a recognition ligand for receptors, which bind these particles. The APOE isoform 2 is a risk factor for age-related macular degeneration; nevertheless, APOE absence in humans and mice does not significantly affect the retina. We found that retinal cholesterol biosynthesis and the levels of retinal cholesterol were increased in Apoe / mice, whereas cholesterol elimination by metabolism was decreased. No focal cholesterol deposits were observed in the Apoe / retina. Retinal proteomics identified the most abundant cholesterol-related proteins in WT mice and revealed that, of these cholesterol-related proteins, only APOA4 had increased expression in the Apoe / retina. In addition, there were changes in retinal abundance of proteins involved in proinflammatory and antiinflammatory responses, cellular cytoskeleton maintenance, vesicular traffic, and retinal iron homeostasis. The data obtained indicate that when APOE is absent, particles containing APOA1, APOA4, and APOJ still transport cholesterol in the intraretinal space, but these particles are not taken up by retinal cells. Therefore, cholesterol biosynthesis inside retinal cells increase, whereas metabolism to oxysterols decreases to prevent cells from cholesterol depletion. These and other compensatory changes underlie only a minor retinal phenotype in Apoe / mice.-Saadane, A. A.

The Interplay between Retinal Pathways of Cholesterol Output and Its Effects on Mouse Retina

“…In Apoe −/− mice, retinal cholesterol was also increased (up to 3-fold). However, cholesterol excess remained mainly unesterified and did not form focal deposits as it was mostly present outside retinal cells on the LP circulating in the intraretinal space [27]. Remarkably, the Apoe −/− retina did not have any abnormalities of clinical relevance in structure or in blood vessels, likely because of the compensatory responses triggered by APOE absence.…”

“…The label-free approach was used as described [27]. Briefly, three biological replicates per genotype (Cyp27a1 −/− Cyp46a1 −/− Soat1 −/− or C57BL/6J, 6-8 month old male mice), each containing two retinas from two different mice, were frozen, cryo-pulverized, and lysed with 3% SDS.…”

“…This was carried out using a 1200EX transmission electron microscope (JEOL Ltd. Tokyo, Japan). Tissue fixation was as described [27,33] and included sequential incubations in 0.1 M Na cacodylate buffer, pH 7.4, containing first 3% glutaraldehyde and then 1% OsO 4 , followed by incubations with 1% tannic acid in 0.05 M Na cacodylate, pH 7.4, and 1% para-phenylenediamine in 70% ethanol.…”

“…To ascertain retinal significance of cholesterol removal by metabolism and LP, we previously comprehensively characterized the retina of Cyp27a1 −/− Cyp46a1 −/− mice and mice lacking APOE, a major apolipoprotein in the retina [22,26,27]. In the former, total cholesterol was increased up to 2-fold, and all of the cholesterol surplus was converted to cholesterol esters, which were mainly accumulated in the photoreceptor outer segments [22].…”

“…Remarkably, the Apoe −/− retina did not have any abnormalities of clinical relevance in structure or in blood vessels, likely because of the compensatory responses triggered by APOE absence. These responses were identified and included changes in the retinal cholesterol biosynthesis and abundance of proteins involved in cellular cytoskeleton maintenance, vesicular traffic, pro-and anti-inflammatory signaling, as well as iron homeostasis [27]. We also generated Cyp27a1 −/− Cyp46a1 −/− Soat1 −/− mice and established that retinal cholesterol esterification is mediated by SOAT1 [22].…”